Balanced pressure drilling

ABSTRACT

This specification discloses a rotary method of drilling a well into the earth. A portion of the drilling mud that is circulated through the well is treated at selected times, which portion is separated into two streams, a high density slurry stream containing a heavy solids phase, and a low density effluent stream containing a light solids phase. The effluent stream is recombined into the drilling mud and the high density slurry stream is stored, thereby decreasing the density of the drilling mud circulated through the well. At other selected times the high density slurry is flowed into the drilling mud to increase the density thereof.

United States Patent Nelson [54] BALANCED PRESSURE DRILLING [72] Inventor: Murrell D. Nelson, Arlington, Tex.

[73] Assignee: Mobil Oil Corporation 7 [22] Filed: I Feb. 23, 1971 211 Appl. No.: r 18,003

[52] US. Cl. ..175/66 [51] Int. Cl. ..E2lb 21/04 [58] Field of Search 175/66, 70, 57

[56] v References Cited 7 UNITED STATES PATENTS 2,632,631 3/1953 Griffin et al. 175166 2,772,073 ll/l956 Johnson 175/66 3,433,312 3/1969 Burdyn et al. ..175/66 3,500,943 3/ l 970 Bingman 175/66 [15] 3,684,038 51 Aug. 15,1972

Primary Examiner-David H. Brown Attorney-William .l. Scherback, William D. Jackson, Henry L. Ehrlich and Sidney A. Johnson ABSTRACT This specification discloses a rotary method of drilling a well into the earth. A portion of the drilling mud that is circulated through the well is treated at selected times, which portion is separated into two streans, a high density slurry stream containing a heavy solids phase, and a low density efiluen't stream containing a light solids phase. The effluent stream is recombined into the drilling mud and the high density slurry stream is stored, thereby decreasing the density of the drilling mud circulated through the well. At other selected times the high density slurry is flowed into the drilling mud to increase the density thereof.

4 Claim, 1 Drawing Figure PATENTEDAUBIS m2 3.684.038

MURRELL D. NELSON INVENTOR ATTORNEY BALANCED PRESSURE DRILLING BACKGROUND OF THE INVENTION This invention relates to the drilling of wells into the earth by rotary drilling techniques and more particularly to balanced pressure drilling techniques wherein the density of the drilling mud in the well is decreased at selected times and increased at other selected times.

In the drilling of wells into the earth by rotary drilling techniques, a drill bit is attached to a drill string, lowered into a well, and rotated in contact with the earth; thereby breaking and fracturing the earth and forming a wellbore thereinto. A drilling mud is circulated down the drill string and through ports provided in the drill bit to the bottom of the wellbore and thence upward through the annular space formed between the drill string and the wall of the wellbore. The drilling mud serves many purposes including cooling the bit, removing cuttings from the wellbore, and supplying hydrostatic pressure upon the formations penetrated by the wellbore to prevent fluids existing under pressure therein from flowing into the wellbore.

Drilling rates have been found to be inversely proportional to the hydrostatic pressure applied at the bottom of the wellbore. Therefore it is desirable during drilling operations to decrease the density of the drilling mud in the wellbore and thereby the hydrostatic pressure applied by the drilling mud column at the wellbore bottom to the lowest value consistent with safe drilling practices.

The total pressure applied at the wellbore bottom when drilling mud is being circulated is made up of two components; (1) the hydrostatic pressure due to the density and height of the drilling mud column in the wellbore, and (2) the pump pressure required to flow the drilling mud at a desired flow rate up the annular space between the drill string and the wellbore wall against the frictional resistance offered to flow bythe drill string and wellbore wall. This total pressure should be approximately equal to or slightly less than the pressure of the fluids in the formations penetrated by the wellbore. However, when drilling mud circulation is to be stopped, as when the drill string is to be removed from the wellbore (referred to as making a trip), it becomes necessary to increase the density of the drilling mud in the wellbore by a sufficient amount to offset the loss of pump pressure in order to prevent formation fluids from flowing into the wellbore. A method of increasing the density of the drilling mud in the wellbore is described in US. Pat. No. 3,507,343 to James A. Gill et al. Gill selects a portion of the liquid in the system and adds to that portion a quantity of coarse, granular weighting material to increase the density of the selected portion by some preselected amount which depends upon the particular circumstances of the drilling operation. Subsequently, the circulating mud is passed through a mechanical treating means which removes and discards the coarse, granular weighting material and thereby restores the drilling mud to substantially its original density.

SUMMARY OF THE INVENTION In accordance with this invention there is provided a method of drilling a well into the earth wherein there is circulated through the well a drilling mud from an active drilling mud system, which drilling mud in the active drilling mud system is comprised of a heavy solids phase and a light solids phase. A portion of the drilling mud is treated to separate the portion into a high densi ty slurry stream containing the heavy solids phase and into a low density effluent stream containing the light solids phase. The low density effluent stream containing the light solids phase is flowed into the active drilling mud system, thereby reducing the density of the drilling mud in the active drilling mud system, and the high density slurry stream is'flowed into storage. sub sequently, the high density slurry stream is flowed from storage into the active drilling mud system to increase the density of the drilling mud in the active drilling mud system.

BRIEF DESCRIPTION OF THE DRAWING The FIGURE is a schematic diagram illustrating the method of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Thisinvention relates to a rotary method of drilling a and chemicals may be added to a liquid carrier in formulating a drilling mud having suitable rheological properties. Barite or other dense materials referred to as weighting materials are added to increase the weight and thereby the density of the drilling mud. Bentonite and other materials such as chemicals which are added to the drilling mud are light as compared to barite and are added to effect other mud properties than the weight of the mud. The drilling mud may be considered to be comprised of a heavy solids phase, made up of the weighting material and a light solids phase made of the remainder of the mud.

In accordance with this invention a portion of the drilling mud being circulated through the wellbore is treated to separate this portion into a high density slurry stream comprised of the heavy solids phase in slurry form and into a low density effluent stream comprised of the light solids phase. The effluent stream is returned tothe drilling mud being circulated through the wellbore, thereby reducing the density of the drilling mud. The high density slurry stream is flowed into storage and at selected times is flowed into the drilling mud being circulated through the wellbore to increase the density thereof.

Referring to the drawing, there is shown a drilling rig 1 that supports a drill string 3 in wellbore 5 by means of swivel 7, traveling block 9, and cable 11. An active drilling mud system is employed whereby drilling mud is circulated by mud pump 17 from mud pit 13, which may be a tank or other suitable means for containing a mud supply, through conduit 15 and drilling mud hose 19, through swivel 7, and down through drill string 3 where it exits through mud ports (not shown) in drill bit 21. The mud then flows up through the annular space 23 formed between drill string 3 and wellbore 5, and

then through conduit 25 and into mud pit 13. A shale shaker 27 may be used to remove from the mud the large cuttings 29 which were carried from wellbore 5 by the mud.

When drilling a wellbore, subsurface formations are penetrated which contain therein fluids under pressure. The pressure of the fluids in the subsurface formations is represented graphically by vector 31. When mud circulation is stopped this pressure must be counterbalanced by the hydrostatic pressure of the mud column in wellbore 5, which hydrostatic pressure is represented graphically by vector 33. A drilling mud having a density sufficient to counterbalance the formation pressure is referred to as being of trip weight. When the drilling mud is flowed from bit 21 up annular space 23 there is a frictional loss, represented graphically by vector 37, which must be overcome by pump pressure in order to circulate the mud. The total pressure at the wellbore bottom during circulation of the drilling mud is the sum of the pump pressure represented by vector 37 and the hydrostatic pressure applied by the mud column. Therefore, the density of the mud column may be decreased below trip weight when mud is being circulated such that the sum of the hydrostatic pressure of the mud column, represented graphically by vector 35, and the pump pressure, represented by vector 37, is equal to the formation pressure represented by vector 31. In accordance with my invention, I treat the drilling mud, conserving all parts thereof, to provide a lessened mud column hydrostatic pressure as represented by vector 35 during drilling operations and a mud column hydrostatic pressure as represented by vector 33 when drilling operations are temporarily stopped as when the drill string is removed from the wellbore when making a trip.

I have developed a method whereby this variation in hydrostatic pressure can be readily achieved while conserving all parts of the drilling mud. The drilling mud in the active drilling mud system is originally formulated to have a trip density, that is, a density sufficient to impose a hydrostatic pressure at the bottom of wellbore in an amount required to control formation pressures when making a trip. A sufficient volume of drilling mud is mixed in mud pit 13 such that wellbore 5 may be completely filled with drilling mud when drill string 3 is removed from the wellbore. In addition, it may become necessary to supply makeup mud as wellbore 5 is deepened or as drilling mud is lost, for example, into the formations or by spillage.

In accordance with an embodiment of this invention, during drilling operations a portion of the mud from the active drilling mud system is removed by pump 39 through conduit 41 to a mechanical separating means 43 where it is separated into two streams; (1) a high density slurry stream containing the heavy solids phase of the drilling mud, and (2) a low density effluent stream containing the light solids phase of the drilling mud. The effluent stream is flowed through conduit 45 and back into mud pit 13 where it is recombined into the active mud system, thereby reducing the density of the drilling mud in the active drilling mud system. The high density slurry stream is flowed from mechanical means 43 through conduit 47 to a storage tank 49 where it is maintained in slurry form for recombining at a selected time with the active mud system.

The high density slurry in tank 49 is available for emergency conditions as for example when an excessively high pressured formation is penetrated. Under such emergency conditions, the high density slurry may be flowed directly into the drill string in order to rapidly apply a high hydrostatic pressure in the wellbore and thereby prevent what is commonly known as a blowout. When it becomes necessary to make a trip, the high density slurry is combined with the drilling mud in the active system to increase the density thereof to trip density. As shown, a mud pump 51 may be used to pump the high density slurry from tank 49 through conduit 53 and opened valve 55 into the active mud system. Conduit 53 may communicate with conduit 15 or mud pit 13, as desired. When it is desired to blend the high density slurry into the active mud system, it is best done by flowing it into the active mud system at a rate such that the amount to be blended is flowed into the system during one circulation cycle of the active mud system. This ensures that the active mud system is uniformly brought up to the desired density.

A particular mechanical means 43 which is capable of being operated to remove the high density slurry from the effluent stream is described in US. Pat. No. 3,400,819 to R. F. Burdyn and in US. Pat. No.

3,433,312 to R. F. Burdyn et al. This mud separator enables the drilling mud to be separated into a high density slurry stream containing weighting materials or heavy solids phase and an effluent stream containing the remainder of the drilling mud. It is highly desirable that the weighting material be removed as a slurry from the portion of the drilling mud being treated. The slurry may be easily pumped into storage and readily pumped back into the active mud system or into the well to meet emergency conditions. Further, the slurry combines and blends readily into the drilling mud in the active mud system. Thus, this method allows the density of the drilling mud used in drilling a well to be decreased at selected times while conserving the expensive fine solids and the weighting materials originally used in making up the mud and further allows these same weighting materials to be reused at other selected times to again increase the density of the drilling mud in use in drilling the well. This method offers the flexibility of tailoring the density of the drilling mud in the active system and thereby precisely controlling the hydrostatic pressure imposed at the bottom of wellbore 5 by the mud column therein. This enables the maximum drilling rate to be obtained consistent with safe drilling practice.

TABLE I presents operational data for reducing the weight of trip mud (weight when drill string 3 is being removed from or run into wellbore 5) by one-half pound per gallon. This is based on a l,000-barrel system with barrels of heavy slurry (at trip weight plus 4.5 pounds per gallon) being extracted and stored while drilling, and 900 barrels of mud (at trip weight minus one-half pound per gallon) remaining in the active mud system. This represents a change in hydrostatic pressure of 26 psi per 1,000 feet depth. The time required in hours and the operational data are based upon a particular system being processed by the mud separator as described in US. Pat. No. 3,400,819 to R. F. Burdyn.

TABLE 1 Time Required to Reduce Mud Weight 1,000 bbls mud reduced b lb/gal) Mud Weight, lb/gal name viscosity of effluent, 20 cp.

cut point of barite will then be about microns, and about 80 percent of barite will be in high density slurry stream.

calculations are based on a gel-barite mud.

What is claimed is:

1. In a method of drilling a well into the earth wherein a drill string is located in said well and an active drilling mud system is employed in the circulation Y of drilling mud between the surface of the earth and the bottom of the well, said drilling mud in said drilling mud system being comprised of a heavy solids phase and a light solids phase, the improvement comprising:

a. treating a portion of said drilling mud in said drilling mud system to separate said portion into a high density slurry stream containing said heavy solids phase and into a low density effluent stream containing said light solids phase;

b. reducing the density of said drilling mud in said active drilling mud system by flowing said low density efiluent stream containing; said light solids phase into said active drilling mud system;

c. concomitantly with step (a) flowing said high density slurry stream into storage; and

d. thereafter increasing the density of said drilling mud in said active drilling mud system by flowing said high density slurry stream from storage into said active drilling mud system. 7

2. The method of claim 1 wherein said high density slurry stream is flowed from storage into said active drilling mud system at a rate to uniformly increase the density of said drilling mud in said active drilling mud system. b

3. The method of claim 1 wherein said high density slurry stream is flowed from storage at a unifonn rate into said active drilling mud system during one complete circulation cycle of said active drilling mud system.

4. The method of claim 1 wherein said high density slurry stream is flowed from storage into said drill string to establish a mud column of said high density slurry in said well thereby increasing the hydrostatic pressure applied in said well. 

1. In a method of drilling a well into the earth wherein a drill string is located in said well and an active drilling mud system is employed in the circulation of drilling mud between the surface of the earth and the bottom of the well, said drilling mud in said drilling mud system being comprised of a heavy solids phase and a light solids phase, the improvement comprising: a. treating a portiOn of said drilling mud in said drilling mud system to separate said portion into a high density slurry stream containing said heavy solids phase and into a low density effluent stream containing said light solids phase; b. reducing the density of said drilling mud in said active drilling mud system by flowing said low density effluent stream containing said light solids phase into said active drilling mud system; c. concomitantly with step (a) flowing said high density slurry stream into storage; and d. thereafter increasing the density of said drilling mud in said active drilling mud system by flowing said high density slurry stream from storage into said active drilling mud system.
 2. The method of claim 1 wherein said high density slurry stream is flowed from storage into said active drilling mud system at a rate to uniformly increase the density of said drilling mud in said active drilling mud system.
 3. The method of claim 1 wherein said high density slurry stream is flowed from storage at a uniform rate into said active drilling mud system during one complete circulation cycle of said active drilling mud system.
 4. The method of claim 1 wherein said high density slurry stream is flowed from storage into said drill string to establish a mud column of said high density slurry in said well thereby increasing the hydrostatic pressure applied in said well. 